eloping germ. After three months the embryo is termed the _foetus_.
Its fluids are then so much more highly organized, that some of them are
tinged with sanguine hues, and thenceforward acquire the characteristics
of red blood. Out of red blood, blood-vessels are formed, and from the
incipient development of the heart follow faint lines of arteries, and
the engineers of nutrition survey a circulatory system, perfecting the
vascular connections by supplementing the arteries with a complete
net-work of veins and capillaries.

THE PLACENTA OR AFTERBIRTH.

Whenever conception occurs, a soft, spongy substance is formed between
the uterus and the growing ovum, called the _placenta_. It is composed
of membrane, cellular tissue, blood-vessels, and connecting filaments.
The principal use of this organ seems to be to decarbonate the blood of
the foetus, and to supply it with oxygen. It performs the same function
for the foetus that the lungs do for the organism after birth. It allows
the blood of the foetus to come into very close contact with that of the
mother, from which it receives a supply of oxygen, and to which it gives
up carbonic acid. This interchange of gases takes place in the placenta,
or between it and the uterus, through the intervening membranes. This
decarbonating function requires the agency of the maternal lungs, for
the purpose of oxygenating the mother's blood.

The placenta is attached to the uterus by simple adhesion. True, in some
instances, morbid adhesion takes place, or a growing together in
consequence of inflammation, but the natural junction is one merely of
contact, the membranes of the placenta spreading out upon the cavity of
the uterus, so that, finally, the former may be entirely removed without
a particle of disturbance or injury to the latter. Formerly, it was
supposed that the placental vessels penetrated into the substance of the
uterus. We know now there is no such continuation of the vessels of the
one into the other. The decarbonation of the blood requires the
placental and uterine membranes to be in contact with each other.

If the union were vascular, the mother's blood would circulate in the
foetal body, and the impulses of the maternal heart might prove too
strong for the delicate organism of the embryo. Besides, the separation
of the placenta from the uterus might prove fatal to both parent and
offspring. The placenta is only a temporary organ, and when its
functions are no lo